The present invention relates to an airflow sensor. More particularly, it relates to an airflow sensor, and associated system and method, for detecting the presence of airflow within an air handling system.
Most air handling systems (e.g., HVAC systems) include one or more components that are controlled as a function of environmental and/or operational parameters. For example, an air conditioner associated with a residential air handling system is commonly activated/deactivated by a thermostat that compares sensed air temperature with a pre-set value. More complex air handling systems employ computer-based, universal environmental control units to integrate and control a number of different components based upon a variety of sensed and operational parameters. To this end, while computer-based control has heretofore been limited to commercial applications, the advent of low-cost microprocessors has made these types of universal control units highly viable for residential platforms.
Regardless of the exact application platform, the presence or absence of forced airflow within the air handling system is a useful parameter utilized in controlling air handling system components, either on an individual basis or as part of a universal control unit. In general terms, most air handling systems include one or more fan/blowers that, when activated, circulate air throughout the living area or occupational environment. The circulating air leaves the fan/blower from a supply side and returns from the controlled space via a return air duct that may draw outdoor air into the home or building via a fresh air intake. The air is then subjected to heating or cooling conditioning (e.g., via a furnace, air conditioner, etc.), and then forced through auxiliary ductwork back to the rooms or offices. Thus, operation of the fan/blower (or similar device) results in the presence or absence of forced airflow within the air handling system. As a result, certain system components that are otherwise controlled as a function of whether airflow is present and/or operation of the fan/blower can utilize airflow detection information to effectuate proper control.
An example of an air handling system component that is preferably operated based upon airflow is an ultraviolet air treatment device positioned to treat air in the return duct. In general terms, ultraviolet air treatment devices include one or more appropriately sized ultraviolet lamps that are positioned within the air handling system""s ductwork. The ultraviolet lamp is normally mercury-based, with the ultraviolet air treatment device including a power supply ballast used to energize the mercury. For residential applications, the ultraviolet air treatment device is mounted to the return air duct, with the lamp(s) protruding inside of the duct itself. During use, the ultraviolet lamp emits ultraviolet energy that destroys unwanted airborne microorganisms otherwise entrained in the airflow. As such, the ultraviolet lamp is most optimally operated when airflow is present, and is deactivated during periods of no airflow to save energy and increase the useful life of the ultraviolet lamp. Other air handling system components, such as air cleaners, humidifiers, etc., are similarly operated based upon the presence or absence of airflow.
One available technique for providing airflow-related information is to wire or electrically connect the particular component and/or universal control unit to the fan/blower, such as with a current sensing relay. This approach is relatively expensive, and is characterized by low reliability and installation complexities. Conversely, available airflow sensors can be located within the air handling system ductwork (such as the return air duct). In general terms, these airflow sensors typically incorporate two temperature sensitive elements (e.g., thermistor, RTD, etc.) and a heating element. During use, one of the temperature sensitive elements is heated by the heating element, while the other is not. Airflow cools the heated temperature sensor, providing a general indication of the presence of airflow.
The above-described airflow sensors are often times formed by potting the requisite elements in a well that is then mounted to protrude into the airflow. Alternatively, the sensor elements are mounted to a circuit board. A housing surrounds the circuit board and mounted components, and includes several small holes or slots that otherwise allow air to interact with at least the heated temperature sensor. Unfortunately, these available airflow sensors are relatively large, and thus present certain installation concerns. Further problems may arise with the potted sensor as this form of packaging causes an associated lag time in the sensing elements that may be too long for acceptable air handling system equipment control. An additional concern common to both forms of sensors is that debris (e.g., dirt, lint, etc.) entrained in the airflow will readily collect on the temperature sensor well or within the holes or slots in the housing, leading to the airflow sensor detection errors/failures. The currently available airflow sensor design is unable to eliminate the debris accumulation problem from a structural standpoint on a cost effective basis or correct for this potential error through programming efforts. Additionally, these sensors are susceptible to errors due to part tolerance deviations, temperature gradients and power supply voltage variations.
The presence or absence of airflow is an important parameter relied upon for optimal operation of certain air handling system components. Unfortunately, currently available techniques, including directly linking to the fan/blower motor or employing a known airflow sensor, are unsatisfactory from both a reliability and cost standpoint. Therefore, a need exists for a low cost airflow sensor that overcomes the debris accumulation issues experienced with current designs, as well as a related system and method for accurately interpreting signals from the airflow sensor to thus detect the presence or absence of airflow.
One aspect of the present invention relates to an airflow sensor for detecting airflow within an air handling system. The airflow system includes a housing, a flexible substrate, and electrical components. The housing defines an internal compartment and a top face. The top face forms an opening into the internal compartment. The flexible substrate includes circuitry traces, and defines a front and a back. In this regard, the substrate is disposed within the compartment such that the back is exposed relative to the opening in the top face. Finally, the electrical components are electrically connected to the circuitry traces and are positioned to extend from the front of the substrate. In other words, upon final assembly, the electrical components extend opposite the opening in the top face of the housing. In one preferred embodiment, the electrical components are embedded into insulating material contained within the housing. Regardless, the electrical components include a first, heated temperature sensor and a second, baseline temperature sensor. The first and second temperature sensors are spaced from one another along the substrate. With this construction, and during use, airflow interfaces with the back of the substrate to cool the first, heated temperature sensor. The extent of this cooling as compared to the second, baseline temperature sensor indicates the presence of airflow. In this regard, the airflow sensor is configured to limit the accumulation of debris along the back of the substrate, thereby minimizing the opportunity for sensor failure. In one preferred embodiment, the temperature sensors are thermistors, and an additional heating element, such as a resistor, is positioned in close proximity to the first temperature sensor to effectuate heating thereof. In a further preferred embodiment, the flexible substrate is a Kapton flexible circuit that is highly smooth, thereby preventing accumulation of debris.
Another aspect of the present invention relates to an airflow sensor system for detecting airflow within an air handling system. The sensor system includes an airflow sensor and a processor. The airflow sensor includes a first, heated temperature sensor and a second, baseline temperature sensor. The processor is electrically connected to the temperature sensors and is adapted to monitor signals therefrom. Further, the processor is adapted to determine current temperatures at the temperature sensors based upon the monitored signals, as well as to compare the determined current temperatures. Finally, the processor is adapted to determine an airflow state within the air handling system based upon the comparison and a rate of change in the difference between the current temperature. In one preferred embodiment, the system incorporates a first order lag filter routine to account for the affects of part tolerances, temperature gradients, and power supply voltage variations. In particular, a lag filter temperature differential (DTLag) that is then compared to the instantaneous value between the heated and unheated sensors (DT), resulting in a temperature rate of change value (DDT). The processor is further adapted to compare the DDT value with one or more predetermined threshold values, and then designate whether airflow is present or absent based upon the comparison. In one preferred embodiment, the processor is adapted to store different threshold values for evaluating whether airflow is on versus whether airflow is off. Finally, in another preferred embodiment, the processor is adapted to utilize a reference variable to confirm the airflow designation evaluation. In this regard, the processor is preferably further adapted to update the reference variable with a current reading upon determining that the air handling system has transitioned from an airflow off state to an airflow on state.
Yet another aspect of the present invention relates to a method for detecting airflow within an air handling system including ductwork. The method includes positioning an airflow sensor within the ductwork. In this regard, the airflow sensor includes a first, heated temperature sensor and a second, baseline temperature sensor. Signals from the temperature sensors are monitored. Based upon these monitored signals, current temperature readings for both of the temperature sensors are determined. Finally, an airflow state within the air handling system is determined based upon a difference between the determined current temperature readings (DT) and a rate of change in DT. In one preferred embodiment, the method includes generating a temperature rate of change value (DDT) based upon a difference between DT and a lag temperature differential value with the lag temperature differential value being generated as a function of DT and a sensor lag parameter. With this one preferred embodiment, the temperature rate of change value is compared with at least one threshold value for determining the presence of airflow. In an even more preferred embodiment, the temperature rate of change value is compared with a first threshold value to determine if airflow is not present, and compared with a second threshold value to determine if airflow is present. In an even more preferred embodiment, the step of determining the presence of airflow further includes comparing the difference between current temperature readings (DT) with a reference parameter that is updated from time-to-time with a current value upon determining that the air handling system has transitioned from an airflow off state to an airflow on state. In another preferred embodiment, the method further includes signaling the determined airflow state to an ultraviolet air treatment device.